1. Field of the Invention
The present invention generally relates to a control apparatus for an electric power steering system in which a steering system of an automobile or a vehicle is provided with a steering assist force generated by a motor. More particularly, the invention relates to a control apparatus for an electric power steering system in which convergence of a yaw rate of a vehicle is ensured.
2. Description of the Related Art
An electric power steering system for assisting a steering system of an automobile or a vehicle by a motor torque operates in such a manner that a steering shaft or a rack shaft is assisted by a driving force of the motor through a speed reducer using a transfer mechanism, such as a gear or a belt. The known electric power steering system performs feedback control of motor current to accurately generate an assist torque (steering assist torque). The feedback control is used for adjusting voltage impressed on the motor so that the difference between a current control value and a motor current detected value decreases. In general, the adjustment of the voltage impressed on the motor is performed by adjusting a duty ratio of pulse width modulation ("PWM") control. Referring now to FIG. 1, a general structure of an electric power steering system is shown. A shaft 2 of a steering wheel 1 is connected to a tie rod 6 of a vehicle wheel through a speed reduction gear 3, universal joints 4a and 4b, and a pinion rack mechanism 5. A torque sensor 10 for detecting a steering torque of the steering wheel 1 is provided on the shaft 2. A motor 20 for assisting a steering effort of the steering wheel 1 is connected through a clutch 21 and the speed reduction gear 3 to the shaft 2. A battery 14 supplies electric power through an ignition key 11 and a relay 13 to a control unit 30 that controls the power steering system. The control unit 30 computes a steering assist command value I of an assist command based on a steering torque T detected by the torque sensor 10 and a vehicle velocity V detected by a velocity sensor 12, and controls the current to be supplied to the motor 20 based on the computed steering assist command value I. The control unit 30 performs ON/OFF control of the clutch 21. The clutch 21 is normally in an ON condition (engaged). However, the clutch 21 is in an OFF condition (disengaged) when the control unit 30 determines that the power steering system is at fault, or the power supply from the battery (voltage Vb) is cut-off by the ignition key 11 or the relay 13.
The control unit 30 consists mainly of a CPU. FIG. 2 illustrates general functions executed in the CPU by a program. For example, a phase compensator 31 does not represent a phase compensator as an independent hardware component; rather, it represents a phase compensating function executed in the CPU. The function and the operation of the control unit 30 are described below. The torque sensor 10 detects a steering torque T and inputs it to the phase compensator 31. The inputted steering torque T is phase-compensated for by the phase compensator 31 to enhance the stability of the steering system. Then the phase-compensated steering torque TA is inputted to a steering assist command value computing unit 32. The vehicle velocity V detected by the velocity sensor 12 is further inputted to the steering assist command value computing unit 32. The steering assist command value computing unit 32 determines the steering assist command value I, which is equivalent to a control target value of the current to be supplied to the motor 20, based on the inputted steering torque TA and the velocity V. Then the steering assist command value I is inputted to a subtractor 30A as well as to a differential compensator 34 of a feed-forward system for increasing a response speed. A deviation (I-i) determined by the subtractor 30A is inputted to a proportional computing unit 35 and to an integral computing unit 36. The proportional output and the integral output are both inputted to an adder 30B. The integral computing unit 36 is used for improving characteristics of a feedback system. The outputs of the differential compensator 34 and the integral computing unit 36 also are inputted to the adder 30B. As a result, all the inputs to the adder 30B add up to a current control value E. The current control value E is inputted as a motor drive signal to a motor drive circuit 37. Finally, a motor current value i of the motor 20 is detected by a motor current detecting circuit 38, which in turn is fed back through the subtractor 30A.
Now referring to FIG. 3, an example of the structure of the motor drive circuit 37 is shown. The motor drive circuit 37 includes a field-effect transistor ("FET") gate drive circuit 371 for driving each gate of FET1 to FET4 based on the current control value E from the adder 30B, an H-bridge circuit including FET1 to FET4, and a booster power supply 372 for driving a high side of FET1 and FET2. The FET1 and FET2 are switched between an ON condition and an OFF condition by a PWM signal of a duty ratio D1, which is determined based on the current control value E, thereby controlling the current Ir actually supplied to the motor 20. The FET3 and FET4 are driven by a PWM signal of a duty ratio D2, which is defined by a predetermined linear function formula (given constants a and b, D2=a.multidot.D1+b) in a region where the duty ratio D1 is of a small value. After the duty ratio D2 has reached 100%, FET3 and FET4 are switched between an ON condition and an OFF condition in accordance with a rotation direction of the motor 20, which is determined based on the sign of the PWM signal.
There are known electric power steering systems that generate a moderate response in quick steerage of a vehicle. An example of such electric power steering systems is shown in Japanese Patent Publication No.45-41246. The Japanese Patent Publication No.45-41246 describes an apparatus which includes a torsion torque sensor for detecting a torsion torque of a steering shaft when turning a vehicle. In response to the output signal of the torsion torque sensor, the apparatus controls a rotation direction and a rotation torque of an electric motor. However, the above known control apparatus for the electric motor has problems as below. When the output of the control apparatus is set at a high level, convergence of a steering wheel in hand-off steerage of the vehicle deteriorates due to inertia of the control system. In addition, when quickly steering the vehicle around a sharp curve, a driver generally feels more comfortable if there is a moderate response to the steering wheel. Nevertheless, the above known electric power steering system does not include a unit for compensating for an assist steering force (power assist) in accordance with the steering speed. Hence, when making a sharp turn around a curve having a small radius, the driver feels insecure because the steering wheel feels too light.
To solve the above problems, a motor control apparatus, such as disclosed in Japanese Patent No.2568817, is provided wherein brake is applied based on the steering angle of a steering wheel. Specifically, the motor control apparatus for an electric power steering system controls rotation direction and rotation torque of an electric motor that provides a steering mechanism with an assist steering force in accordance with a command signal based on the output signal of a torsion torque sensor for detecting a torsion torque of the steering system. The motor control apparatus includes a detector for detecting a steering angular velocity in the steering system, a steering angle phase-compensating command unit for generating a damping signal, which defines rotation torques in both the steering forward direction and the backward direction, in accordance with the steering angular velocity, and a drive control unit for controlling the rotation direction and the rotation torque of the electric motor in accordance with a command signal which is the sum of the damping signal and the command signal determined based on the torsion torque signal of the steering system.
However, the above known apparatus generates a rotation torque, in response to the steering angular velocity, in the direction opposite to the steering forward direction, and brakes a change in the steering angle, thereby leading to the following problems. Specifically, because the apparatus directly brakes change in the steering angle, there is a risk that yawing of the vehicle diverges. In addition, the yawing of the vehicle is asynchronous with the steering angle, which causes an unnatural steerage feeling for a driver. Further, because the brake is directly applied to resist the change in the steering angle, the rate of convergence responding to the effort on the steering wheel is slow, during which the vehicle moves laterally, thus resulting in a dangerous situation.